In an automobile assembly line, a work area is used where self-propelled travelling carriers loaded with vehicle bodies are moved at a fixed low operating speed with respective work floors thereof being continuous in a moving direction and at the same time required operations are performed on the loaded vehicle bodies. On the occasion when the self-propelled travelling carriers are moved on their own in such a work area, a movement permission needs to be given from the ground to all of the travelling carriers within the work area and the moving at the operating speed needs to be continued during the period when the movement permission is given. If a situation that retreating of the travelling carriers from the work area cannot be continued occurs or if a travelling carrier to be entered into the work area is delayed and the entire region of the work area cannot be provided with the predetermined number of travelling carriers, the movement permission to each travelling carrier needs to be canceled from the ground to stop the travelling carriers on the spot. In performing such control, wired or wireless data communication means capable of communicating between all of the travelling carriers within the work area and the ground are employed in general although no patent documents can be disclosed.